A light-emitting diode (LED) is a p-n junction diode which emits light when activated. For example, when a suitable voltage is applied to the leads, electrons are able to recombine with electron holes within the device, releasing energy in the form of photons, with the color of the light (corresponding to the energy of the photon) determined by the energy band gap of the semiconductor. This effect is called electroluminescence. Light-emitting diodes are used in applications as diverse as aviation lighting, automotive headlamps, traffic signals, camera flashes, etc.
LEDs can be composed of different materials. A recent advancement, for example, is the use of gallium-nitride-on-silicon (GaN-on-Si) to produce LEDs. This combination avoids the typical costly sapphire substrate. Also, manufacturing large sapphire material is difficult, while large silicon material is cheaper and more abundant.